If mollusks have such heavy shells to drag around with them, how have they spread all over the ocean? -Michel W.

Dear Michel,

Mollusks, from land snails and slugs to oysters and mussels in the sea, have a few things in common. They have a head. They have a soft middle part that holds their organs. Then, some have a muscle that’s known as a “foot.” » More …

It’s Shark Week, so I made a visit to my friend Jon Mallatt. He’s a Washington State University biologist who has studied the jaws of ancient sharks.

Dr. Universe: What is something a lot of people might not know about sharks?

Jon Mallatt: Some of them, such as tiger sharks, cat sharks, and even great white sharks, have quite large brains—relative to their body weight— and are intelligent. They are not “primitive” animals. The shark relatives, Manta rays and devil rays, have even larger brains than any shark.

Dr. U: How long have sharks been around, anyways?

JM: At least 420 million years and maybe 460. It is hard to tell for sure because they do not have bone, so their skeletons usually are not preserved as fossils. Hard teeth and scales do fossilize, but may not tell us enough. Their teeth evolved from the tiny scales on their rough skin called denticles. But the first sharks may have had no teeth, or their teeth were the same as these denticles so we would not recognize them.

Older fishes without jaws evolved before sharks did, and the oldest fish of all is 520 million years old. This was not long after the first great evolution of all animals at 540 to 520 million years ago. Thus, sharks are very old, maybe appearing only 100 million years after the first animals.

Dr. U: What are some of the coolest things you’ve learned about sharks in your job as a scientist?

JM: They do not get cancer very often. They build up a lot of urea, or “pee poison” in their blood and it does not hurt them. They can feel fast, sharp pain, but not slow burning pain so they probably do not suffer. They show how successful the vertebrates became as predators after fish first evolved jaws about 460 million years ago.

Dr. U: I heard some sharks lose 1,000 teeth in their lifetime. Why do they lose them?

JM: The reason they lose teeth is that most sharks bite really hard when they chomp on their prey. That means their teeth break, come loose, and are lost, so it is good to have replacement teeth throughout life. Sharks also have lots of rows of new teeth covered by the gum inside the jaw, which are always growing up to replace the exposed teeth when they shed.

Dr. U: Woah! Okay, one last question. I think the cat shark is my favorite. Do you have a favorite kind of shark?

JM: I like the six and seven-gilled sharks, which are related. These include the cowshark, Hexanchus, and the frill-shark, Clamydoselachus. Scope them out online. Especially the frill shark: looks a like a cross between an eel and a dragon. They may represent the oldest line of living sharks and they have some primitive features. They are mysterious too, from the deep ocean.